The present invention is in the area of methods for formulating devices for tissue regeneration, which uses computer-aided design (CAD) in combination with solid free-form fabrication technology to form vascularized polymeric structures which can be implanted, seeded with cells, and form new tissue.
Strategies for regenerating tissue are being developed in response to a range of clinical needs, including replacement of damaged or genetically absent metabolic function from tissues such as liver, pancreas and pituitary tissue, and repair or restructuring of damaged or malformed connective tissues such as bone, cartilage and skin. Unlike blood or bone marrow tissues which can be regenerated by intravenous injection of cells, regeneration of most tissues requires a template.
New therapies for tissue regeneration include approaches in which cells are transplanted into a patient along with a device, and approaches in which a device is implanted next to normal tissue and guides the growth of that tissue into a new region. An example of the latter is a bone regeneration device placed into a fracture site, which guides growth of bone tissue into the fracture. Various materials are used to fabricate inorganic or inorganic/polymer matrices for bone regeneration. These include the coralline replaniform hydroxyapatite, which is essentially an adapted coral as described by Martin, R. B., et al., "Bone ingrowth and mechanical properties of coralline hydroxyapatite one year after implantation," Biomaterials, 14:341-348 (1993), and devices which incorporate a cellular component, as described by U.S. Pat, Nos. 4,620,327, 4,609,551, 5,226,914 and 5,197,985 to Arnold Caplan. Composite materials have also been described; however, they have been used primarily for fixation devices, and not bone ingrowth. See, for example, Boeree, N. R., et al., "Development of a degradable composite for orthopedic use: mechanical evaluation of an hydroxyapatite-polyhydroxybutyrate composite material," Biomaterials, 14:793-796 (1993).
Tissue engineering has emerged as a scientific field which has the potential to aid in human therapy by producing anatomic tissues and organs for the purpose of reconstructive surgery and transplantation. It combines the scientific fields of materials science, cell and molecular biology, and medicine to yield new devices for replacement, repair and reconstruction of tissues and structures within the body. Many approaches have been advocated over the last decade. One approach is to combine tissue specific cells with open porous polymer scaffolds which can then be implanted. Large numbers of cells can be added to the polymer device in cell culture and maintained by diffusion. After implantation, vascular ingrowth occurs, the cells remodel, and a new stable tissue is formed as the polymer degrades by hydrolysis. The diffusion distance for nutrients in vivo is only about 0.2 mm. Thus, a challenge in engineering structures thicker than 0.5 mm is to ensure an adequate supply of blood-borne nutrients, including oxygen (Cima, et al., J. Biomechan. Eng. 113, 143-151 (1991)).
A number of approaches have been described for fabricating tissue regeneration devices for either in vitro or in vivo growth of cells. Polymeric devices have been described for replacing organ function or providing structural support. Such methods have been reported by Vacanti, et al., Arch. Surg. 123, 545-549 (1988), U.S. Pat. No. 4,060,081 to Yannas, et al., U.S. Pat. No. 4,485,097 to Bell, and U.S. Pat. No. 4,520,821 to Schmidt, et al. In general, however, the methods used by Vacanti, et al., and Schmidt, et al., can be practiced by selecting and adapting existing polymer fiber compositions for implantation and seeding with cells, while the methods of Yannas and Bell produce very specific modified collagen sponge-like structures.
Tissue regeneration devices must be porous with interconnected pores to allow cell and tissue penetration. Factors such as pore size, shape and tortuosity can all affect tissue ingrowth but are difficult to control using standard processing techniques. U.S. Ser. No. 08/200,636, "Tissue Regeneration Matrices By Solid Free-Form Fabrication Techniques" filed Feb. 23, 1994 by Linda G. Cima and Michael J. Cima, described the use of solid free form fabrication techniques, especially three dimensional printing of polymer powders, to form matrices which could be seeded with dissociated cells and implanted to form new structures. The advantages of the solid free form methods to construct specific structures from biocompatible synthetic or natural polymers, inorganic materials, or composites of inorganic materials with polymers, where the resulting structure has defined pore sizes, shapes and orientations, particularly different pore sizes and orientations within the same device, with more than one surface chemistry or texture at different specified sites within the device, is readily apparent. However, the devices still have a major limitation: ingrowth of new tissue to form blood vessels which sustain the implanted cells must occur at the right time relative to the increasing cell density within the matrix to sustain the implanted cells, and other tissues must not encapsulate or infiltrate the matrix to choke out or otherwise destroy the implanted cells.
It is therefore an object of the present invention to provide methods and compositions for the preparation of polymeric matrices with complex, temporal and spatial patterns for use in tissue regeneration, which have predesigned vasculature, allowing the matrix to be implanted, connected to the existing blood supply, and immediately function as a new tissue or organ.
It is another object of the present invention to provide methods for culturing matrices seeded with cells so that lumens or other vessels are formed.
It is still another object of the present invention to provide matrices for tissue generation having pre-existing lumens and ducts within the matrix for exocrine, excretory, and other functions associated with normal tissue in vivo.